1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, and a phase modulation device, particularly to a crystallization apparatus which irradiates a polycrystal or amorphous semiconductor film with laser light having a predetermined light intensity distribution to produce a crystallized semiconductor film.
2. Description of the Related Art
A thin-film transistor (TFT), for example, for use in a switching device which controls a potential applied to a pixel of a liquid crystal display (LCD) has heretofore been formed in an amorphous silicon layer or a poly-silicon layer.
The poly-silicon layer has an electron or hole mobility higher than that of the amorphous silicon layer. Therefore, in the case where the transistor is formed in the poly-silicon layer, switching speed is increased and, consequently, display response is accelerated as compared with the case where the transistor is formed in the amorphous silicon layer. Therefore, it is possible to constitute a peripheral LSI with thin-film transistors with the advantage that design margins of other components can be reduced. Furthermore, in the case where peripheral circuits such as a driver circuit and DAC are incorporated on the same substrate as the display, it is possible to increase operation speeds of the peripheral circuits.
Polycrystal silicon is constituted of an aggregation of crystal grains, and the mobility of electrons or holes is lower than that for single-crystal silicon. However, in the case where a large number of thin-film transistors are formed in the poly-silicon layer, fluctuations of crystal grain boundary number in a channel portion raise a problem. To solve the problem, in recent years, a crystallization method of producing crystallized silicon having large grain diameters has been proposed in order to enhance the mobility of electrons or holes and to reduce the fluctuations of the crystal grain boundary number in the channel portion.
A phase modulation excimer laser annealing process (phase modulation ELA process) has heretofore been proposed as the crystallization method (Surface Science, Vol. 21, No. 5, pp. 278 to 287, 2000). According to this method, a phase shifter is disposed parallel to and in the vicinity of the polycrystal or amorphous semiconductor film, and the semiconductor film is irradiated with excimer laser light via the phase shifter to crystallize the semiconductor film.
In the phase modulation ELA process, a light intensity distribution having an inverse peak pattern (the light intensity becomes minimum at the center, and the light intensity rapidly increases toward the periphery) in which the light intensity is minimized at the point corresponding to the phase shift steps of the phase shifter is generated, and the polycrystal or amorphous semiconductor film is irradiated with the light having the light intensity distribution having the inverse peak pattern. As a result, a temperature gradient is generated in a molten region in accordance with the light intensity distribution, a crystal nucleus is formed in the portion which first solidifies, corresponding to the point at which the light intensity is minimized, and crystals grow in a lateral direction toward the periphery from the crystal nucleus (hereinafter referred to as “lateral growth” or “lateral-direction growth”). Accordingly, single-crystal grains having large grain diameters are produced.
In Jpn. Pat. Appln. KOKAI Publication No. 2000-306859, a technique has been disclosed in which the semiconductor film is irradiated with light having the light intensity distribution having the inverse peak pattern produced via a phase shift mask (phase shifter) to crystallize the film. Further in Inoue et al., Journal of Papers of the Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 8, pp. 624 to 629, August 2002, a technique has been disclosed in which the semiconductor film is irradiated with light having a light intensity distribution of a concave pattern plus inverse peak pattern produced by combining the phase shifter with a light absorption distribution.
As disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2000-306859, the light intensity distribution of the inverse peak pattern is formed in the portion corresponding to a phase shift portion in a conventional technique in which the phase shifter is used to form the light intensity distribution of the inverse peak pattern. However, since the light intensity does not increase linearly, growth of the crystals easily end midway. Since a surplus irregularity distribution is easily generated in the periphery of the light intensity distribution having the inverse peak pattern, the light intensity distribution having the inverse peak pattern cannot be arranged into an array, or crystal grains cannot be generated in the array.
It is to be noted that when the angle distribution of illuminative light with respect to the phase shifter is adjusted, or the arrangement position of the phase shifter is designed, it may be possible to bring an obtained light intensity distribution close to an ideal distribution. However, the design cannot be analytically performed with a prospect. Even if the analytic design can be realized, it is expected that considerably complicated conditions are obtained.
It is to be noted that, as disclosed in Inoue et al., Journal of the Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 8, pp. 624 to 629, August 2002, the light intensity distribution of the concave pattern plus inverse peak pattern for the crystallization can be obtained in the conventional technique in which the phase shifter is combined with the light absorption distribution. However, it is difficult to grow the crystals having large grain diameters in the lateral direction. It is generally difficult to form a film having a light absorption distribution which continuously changes. Especially, when a film to be crystallized is irradiated with light having a very high intensity for the crystallization, deterioration of film materials of the film having the light absorption distribution is unfavorably easily caused by thermal or chemical changes by light absorption.